When a set of rigid or flexible bodies are joined together by means of pins or other kinematic joints a linkage is established. Linkages are used to transmit power and information. They also may be employed to make a point on the linkage follow a prescribed path. Linkages are also used to produce an angular or linear displacement.
Often one wants a machine to move, stop, do something and move again. Servomechanisms have been developed that perform the above. However, servomechanisms are relatively expensive. Numerous devices have been developed to cause intermittent motion providing an indexing function. For example, Geneva mechanisms have been developed and meet the above need to a limited extent. The following references Geneva mechanisms, but similar comparisons can be made with a variety of indexing and motion transfer devices.
A Geneva mechanism is a type of linkage that locks an output shaft at particular angular orientations. Between the locking times the output shaft will rotate. Typically, a Geneva mechanism consists of a driver wheel and a driven wheel receiving one increment of motion for each revolution of the driver. The driver wheel contains a pin or roller while the driven wheel contains three or more equally spaced radial slots. As the driver wheel rotates, the pin attached to the driver wheel engages a slot on the driven wheel and rotates the driven wheel one increment of motion. After the pin leaves the slot, the driven wheel remains locked until the driver pin enters the next slot on the driven wheel. Thus, the constantly rotating driver wheel produces intermittent rotation in the driven wheel.
Geneva mechanisms are very useful for low speed applications where equal increments of motion are required. However, Geneva mechanisms produce jerk (the first derivative of acceleration) which causes an impact force.
Another disadvantage of Geneva mechanisms is that Geneva mechanisms can experience backlash because of the clearance required between pins and slots.
Hence, Geneva mechanisms are not practical at high speeds.
A further disadvantage of Geneva mechanisms is that they are limited to producing certain types of motion e.g., motions are generally modified harmonic in nature and are a function of pin and slot geometry. Also, since the radial slots must be equally spaced only equal increments of angular displacement take place.